Polyesters typified by polyethylene terephthalate (PET) are excellent in transparency and mechanical properties, and accordingly are widely used as materials for bottles and sheets. A polyester is usually obtained by performing an esterification and/or transesterification reaction between a dicarboxylic acid component and a diol component, and then performing a polycondensation reaction while taking out the diol component out of the reaction system under reduced pressure.
For example, by using a terephthalic acid component and an ethylene glycol component or a 1,4-butanediol component, a homopolymer of PET or polybutylene terephthalate (PBT) is obtained; however, by adding a dicarboxylic acid component and/or a diol component other than these monomer components constituting PET or PBT, a copolymerized polyester having various properties can be obtained. In particular, flexibility can be imparted by copolymerizing polyoxytetramethylene glycol with a polyester such as PET or PBT, and by further copolymerizing another monomer with this copolymer and making the polyester amorphous, a soft copolymerized polyester similar to a soft vinyl chloride resin can be obtained (see, for example, Patent Literature 1 to Patent Literature 3).
However, because the glass transition temperature (Tg) of such an amorphous soft copolymerized polyester is often room temperature, namely, 25° C. or lower, when the amorphous soft copolymerized polyester is formed or molded and exposed to a temperature of Tg or higher over a long term, there has occurred a problem that the bleeding-out of the oligomer occurs in the soft polyester resin, and a portion of the oligomer turns into fine crystals, leading to loss in the transparency and surface gloss of formed or molded products. In addition, the amorphous resin has a reduced heat resistance, and as a result, there has been another problem that the formed or molded products or pellets for forming or molding are liable to stick to each other.
In contrast, for example, in Patent Literature 4, it was reported that the bleeding-out of the oligomers is reduced by further copolymerizing 5-sodium sulfoisophthalic acid or a diester derivative thereof with a polyester resin in which polyoxytetramethylene glycol is copolymerized. However, the method of Patent Literature 4 can provide a polyester resin in which the bleeding-out over time is reduced, but the resulting polyester resin is insufficient in heat resistance, and therefore, the problem of sticking between formed or molded products or pellets has not been solved.